Former colonial power France, with which the junta broke military ties shortly after taking power on July 26, said it would "firmly" back whatever course of action the ECOWAS bloc took after the Sunday deadline expired.

"The future of Niger and the stability of the entire region are at stake," the office of French Foreign Minister Catherine Colonna said, after she met Niger's prime minister, Ouhoumoudou Mahamadou, in Paris.

ECOWAS military chiefs of staff have agreed a plan for a possible intervention to respond to the crisis, the latest of several coups to hit Africa's Sahel region since 2020.

"All the elements that will go into any eventual intervention have been worked out," ECOWAS commissioner Abdel-Fatau Musah said on Friday.

These included "the resources needed, and including the how and when we are going to deploy the force", he added.

"We want diplomacy to work, and we want this message clearly transmitted to them [the junta] that we are giving them every opportunity to reverse what they have done," Musah said.

Niger has played a key part in Western strategies to combat jihadist insurgencies that have plagued the Sahel since 2012, with France and the United States stationing around 1,500 and 1,000 troops in the country, respectively.

Yet anti-French sentiment in the region is on the rise, while Russian activity, often through the Wagner mercenary group, has grown. Russia has warned against armed intervention from outside Niger.

- 'Error of judgement' -

The coup "is an error of judgement that goes totally against the interests of the country", French Armed Forces Minister Sebastien Lecornu told AFP in an interview on Saturday.

He noted that Niger, one of the poorest countries in the world, relied heavily on foreign aid that risked being pulled if President Mohamed Bazoum is not reinstated as chief of state.

The junta has warned it will meet force with force.

Mali and Burkina Faso, where military juntas have taken power since 2020, have also warned that any regional intervention would be tantamount to a "declaration of war" against them.

Bazoum, 63, has been held by the coup plotters with his family in his official Niamey residence since July 26.

In a column in The Washington Post on Thursday -- his first lengthy statement since his detention -- Bazoum said a successful putsch would "have devastating consequences for our country, our region and the entire world".

Bazoum, who in 2021 won an election that ushered in Niger's first-ever transfer of power from one civilian government to another, urged "the US government and the entire international community to help us restore our constitutional order".

Nigeria has cut electricity supplies to its neighbour Niger, raising fears for the humanitarian situation in the country, while Niamey has closed the vast Sahel country's borders, complicating food deliveries.

Washington said that it had suspended some aid programmes but pledged that "life-saving humanitarian and food assistance will continue".